1. Field
The present invention relates to devices for grading terrain and more particularly to the improved control of tractor drawn blades, such as box blades.
2. Prior Art
There have been a number of prior art devices designed to produce level terrain by using different blades and different methods of attachment of the blade to the tractor. In particular, different methods have been used to actuate the blade to compensate for variations in the initial terrain. Roughness of the terrain and especially high and low areas in the initial terrain typically cause the tractor and the attached blade to move in a vertical and sometimes horizontal direction, making level grading difficult. Constant adjustments of the controls by the tractor operator are usually required to achieve the desired level terrain. To more clearly illustrate this problem, three prior art inventions designed to grade or contour terrain are listed below along with the difficulties encountered with each of these inventions.
Generally box blade are connected to a tractor by way of what is referred to as a three point hitch such as hitch 2 shown in FIG. 1. The box blade 3 is connected to the tractor 1 by the hitch 2. This hitch has three support members which extend from the tractor to the box blade. If it is necessary to raise or lower the box blade, an hydraulic cylinder 4 connected between the tractor and the hitch raises and lowers the hitch, but the hitch rotates about a point 2A and can place the lower surface of the blade 3A unintentionally at a pitch angle with the horizontal plane, as shown by the position of the blade shown in the dashed lines of FIG. 2. The pitch angle has been exaggerated in this Figure for illustrative purposes, but the problem it produces is real and significant even though the actual pitch angles may be small. This unintentionally produced pitch angle requires the tractor operator to supply constant corrections to compensate. In general, the production of the unintentional pitch angle makes grading for the operator more difficult and time consuming.
U.S. Pat. No. 4,299,290 employs a set of wheels at the rear of a blade to aid in supporting the blade above the existing terrain. Hydraulic systems are used to further adjust the front and rear of the blade above the terrain.
The problem encountered with this arrangement is the hydraulic system used to control the blade must be continuously controlled manually. A particular position for the box blade cannot be simply "found" and "locked" in place. It is difficult to hold the lower surface of the box blade in the horizontal plane and also hold it at a particular height of say one-fourth of an inch above the surface of the terrain.
This system also suffers from the effects of an initially rough terrain because the blade does not instantaneously level the terrain before the trailing wheels pass over it. Consequently, the wheels follow the initial contours of the terrain and tend to move the blade away from its desired position.
The blade shown in this patent is a box blade which includes sides and a partially open bottom that takes on soil taken when passing over high spots and empties soil when passing over low spots. These filling and emptying actions take place as long as the blade is held at a desired elevation and the lower surface of the blade remains generally parallel with the horizon plane so that the blade picks up the correct amount of fill and unloads it where it is needed. To keep the lower surface of the blade in this position is difficult because the movement of the wheels of both the tractor and the wheels attached to the blade can throw the blade off from its desired position. Also, the hydraulic systems used to correct the position of the blade does not ordinarily include controls designed to automatically compensate for such problems. As a result, the operator must continually adjust the controls to compensate for these problems. This system does not inherently tend to keep the lower surface of the box blade in a generally horizontal position, which would greatly reduce the complexity of control. An additional problem is the wheels at the rear of the blade prevent the tractor from coming close to a wall. This prevents this arrangement from being applied to complete many areas and thereby generally reduces its usefulness.
U.S. Pat. No. 4,919,212 shows a method of connecting a dozer blade to the front of a tractor. There are two hydraulic controls used to position the blade; however, movement of one of the controls tips the blade either to the right and left and the other moves the blade up and down. There is no single control to keep the blade in the same relative position with respect to the terrain when the tractor runs over a high spot in the terrain. The lower surface of the blade is not inherently maintained in position to grade parallel to the horizon plane.
U.S. Pat. No. 5,511,326 shows a ditcher which has a parallelogram-like linkage connecting the ditcher blade to the tractor. However, all the members of the parallelogram are not rigid. One member is an hydraulic cylinder which does not permit maintaining the lower surface of the blade in the horizontal plane because the movements in the hydraulic cylinder changes its length and destroys the normal functioning of the parallelogram.
What is needed in this field to overcome these disadvantages is a system that permits level grading with a box blade without constant controlling and without the need for wheels on the rear of the box blade which restricts the usefulness of the system.